This invention relates to a method for making an aluminum lithographic plate which is more commonly identified as lithoplate. More particularly, it relates to an improvement in the method of making a workpiece having improved grainability.
Lithography is defined as the process of printing from a plane surface such as a stone or metal plate on which the image to be printed is ink-receptive and the blank area ink-repellant. The stone or metal plate is referred to as lithoplate, but for purposes of discussing this invention and its background, lithoplate will always refer to metal, or more particularly, an aluminum alloy. In addition, although the term "lithoplate" incorporates the word "plate", lithoplate is not necessarily a plate. Rather, lithoplate is used to describe products that might otherwise be considered to be sheet or foil.
The ink-receptive and ink-repellant areas on lithoplate are developed by subjecting the plate to contact with water in the printing press. The image area is hydrophobic or water-repellant, and the non-image area is hydrophilic or water-retentive. The inks used for printing are such that they will not stick or adhere to wet surfaces and, thus, when the lithoplate is contacted with an ink-laden roller, ink is transferred only to the image area.
It is evident that the quality or suitability of a lithoplate for printing is directly related to the hydrophobic and hydrophilic characteristics of the image and non-image areas. It has long been known that uniform roughening of the surface by a process known as "graining" is advantageous in developing both the hydrophobic and hydrophilic areas.
To make the image area, a lithoplate workpiece is coated with a hydrophobic light-sensitive coating material. This material also is resistant to attack or dissolution from acids until it is exposed to light and is commonly called a resist. After the workpiece has been coated with the resist, a negative having the desired image thereon is overlaid on the resist-coated workpiece and exposed to light. In the non-image area, the light causes a reaction in the resist which makes it soluble in acid and, thus, after exposure to light, the plate is contacted with acid to remove the resist in the non-image area. Hydrophobic resist material remains, therefore, only in the image area, and the underlying grained metal surface is advantageous in bonding the resist to it. In the non-image area, with the resist removed, the grained surface is advantageous in enhancing the water retention character of the surface.
Originally, graining of the workpiece was accomplished mechanically by ball graining or brushing. In ball graining, a slurry of steel balls and abrasive material is agitated on the workpiece with the extent of roughening controlled by such things as the type of abrasive, number of balls, speed of agitation, etc. In brush graining, brushes are rotated or oscillated over the surface covered with an abrasive slurry. Mechanical graining usually requires cleaning the plate to make it suitable for further processing. Typically, cleaning is accomplished by immersion in a commercial caustic type solution. It is evident that uniformity and quality of the roughened surface is difficult to control with such methods. In addition, mechanical graining may be relatively slow and costly.
Because of difficulties in mechanical graining, the constant growth of lithographic printing, higher operating speeds of modern printing presses, need for longer lithoplate life, etc., increasing attention has been given to chemical and electrochemical methods of graining. By these methods, the grain is produced by a controlled etching of the surface by the use of chemicals alone or the combination of passing current through a chemical solution. U.S. Pat. Nos. 4,301,229, 4,377,447 and 4,600,482 are cited as examples of many that are directed to electrochemically graining. Whether mechanically grained or electrochemically grained, lithoplate workpieces have certain requirements in common. Lithoplate is used in light gauges, such as 0.008 or 0.012 inch, for example, and by the nature of its use, it must be relatively flat. The surface should be free of imperfections such as deep gouges, scratches and marks which would interfere with the production of a uniform grained surface. From the standpoint of economics or commercial utilization in making aluminum lithoplate, it is desirable that it be produced from an aluminum alloy which can be rolled to the light gauges noted above at reasonable production rates and reasonable levels of recovery or scrap loss. It is also desirable that the alloy from which the lithoplate is made be one which produces reasonably uniform grain when rolled to finished gauge.
In addition, it has become a common practice to apply an anodized finish to the grained surface, whether mechanically or electrochemically produced. It is desirable, therefore, that the aluminum alloy and fabricating practices used to make lithoplate be such that the sheet responds well to anodizing; that is, be uniform in color and relatively free from streaks.
Heretofore, a number of aluminum alloys have been tried and evaluated for the commercial production of lithoplate to be mechanically grained, and the most widely used alloys today are 3000 and 1000 series commercial Aluminum Association alloys (3XXX and 1XXX). In consideration of all of the foregoing lithoplate requirements, these alloys have been determined to be the best from the sheet manufacturer and lithoplate maker or user point of view. With respect to electrochemical graining, however, the response of an aluminum alloy to the particular chemicals employed is obviously an important factor, and these alloys are generally not preferred for graining by such methods.
It would be desirable, therefore, to provide a workpiece fabricated from non-heat treatable alloys such as 3000, 1000 and 5000 series commercial Aluminum Association alloys which would be suitable for graining by either a chemical or electrochemical method.